1. Field of the Invention
This invention relates to polyurethane lacquers for the grain-masking and high-gloss coating of leather.
2. Description of the Prior Art
In addition to dressings which protect leather and favorably modify its properties without influencing its character, the leather industry also uses lacquers which are intended visibly to alter or even mask the grain surface of the leather. Lacquer layers such as these were originally applied solely to black leather and were intended to emphasize the depth of its black color and to provide the leather with a high-gloss surface. Nowadays, this method of finishing leather is widely used for decorative purposes.
Accordingly, the coating materials used must satisfy increasingly more stringent requirements. As in many other fields involving surface finishing, polyurethanes ("PUR") also occupy a leading position among leather lacquers because of their outstanding properties.
For some considerable time, leather has been universally lacquered using PUR lacquers based on two-component systems. In two-component systems, a mixture of polyisocyanates and polyhydroxyl compounds is initially applied, the actual polyurethane being directly produced on the surface of the leather on completion of coating. Hitherto, it has only been possible by this process to produce high-gloss films using hydroxyl polyesters. This process is also attended however, by serious disadvantages:
(1) The pot life of the mixture of the isocyanate and hydroxyl compound is limited and the flow properties gradually change on standing.
(2) During the drying of the lacquer layers, the moisture in the surrounding atmosphere often has an unpredictable effect on the properties of the lacquered leather.
(3) The polyurethane layers applied frequently show inadequate thermal stability under load which restricts their processing on heat-forming machines.
Accordingly, there has been no shortage of attempts to eliminate these disadvantages. One possible approach is to use air-drying or moisture-drying one-component lacquers which, although capable of virtually indefinite storage in sealed containers, dry quickly after application to leather. Lacquers such as these still contain free isocyanate groups and generally belong to the class of socalled prepolymers.
Accordingly, it was obvious to eliminate the disadvantages of the two-component systems, particularly their inadequate stability in storage, by using reactive one-component lacquers such as these. Efforts in this direction, however, have previously been unsuccessful. More specifically, the known one-component lacquers have the following serious disadvantages:
(1) In contrast to the two-component systems which also contain hydroxyl polyesters, polyester prepolymers leave the final lacquer coating with inadequate gloss and insufficient body and, for this reason, are inferior to the corresponding two-component lacquers.
(2) Although prepolymers containing hydroxyl polyethers show excellent gloss, they also have a property which makes them particularly unsuitable for dark colored leather. Natural and synthetic fats migrate from the leathers through the lacquer to its surface where they form cloudy deposits which reduce gloss and, in addition to lightening its color, leave the leather with an unpleasant feel. This disadvantage cannot be obviated by conventional methods, for example by the addition of silicone compounds.
By virtue of the one-component leather lacquers according to the present invention described in the following, it is now possible to eliminate all these disadvantages and to obtain systems which are in no way inferior to conventional two-component lacquers in regard to their properties and, in addition, show additional advantages in regard to shelf life and processing. This is because it has surprisingly been found that a mixture of hydroxyl polyesters and special hydroxyl polyethers as the polyol component leads to prepolymers which give a coating characterized by high gloss, high flexural strength, excellent body and a sealing effect on leather additives.